Since carbon black has hydrophobicity and low wettability with water, it is extremely difficult to stably disperse carbon black in an aqueous medium at a high concentration. This is because the surface of carbon black has only a small number of functional groups (e.g., hydrophilic hydrogen-containing functional groups such as a carboxyl group and a hydroxyl group) having high affinity with water molecules. Various attempts have been made to improve the water-dispersibility of carbon black by oxidizing the carbon black to produce hydrophilic functional groups on the surface of the carbon black.
For example, JP-A-48-018186 discloses a method which oxidizes carbon black with a hypohalite aqueous solution, and JP-A-57-159856 discloses a method which oxidizes carbon black by a low-temperature oxygen plasma.
A method of producing an aqueous ink has also been proposed in which the water-dispersibility of moderately oxidized carbon black is improved using a coupling agent, a surfactant, or the like (e.g., JP-A-4-189877). However, it is difficult to stably maintain the dispersibility of carbon black over a long period of time because the surfactant or the like is oxidized or decomposed due to a change in temperature or a change with time.
As a method of surface-treating carbon black while improving dispersibility, a method has been proposed which finely atomizes carbon black in water utilizing glass beads and oxidizes the carbon black with a hypohalite. However, the atomization effect is diminished due to buoyancy applied to the glass beads in water. Moreover, since active sites are rarely formed, it is difficult to uniformly produce functional groups on the surface of the carbon black.
JP-A-08-003498 discloses an aqueous pigment ink containing water and carbon black having a surface active hydrogen content of 1.5 mmol/g or more, and a method of producing an aqueous pigment ink containing water and carbon black which includes (a) providing acidic carbon black and (b) further oxidizing the acidic carbon black in water using a hypohalite. JP-A-08-319444 discloses a method of producing an aqueous pigment ink which includes finely dispersing carbon black having an oil absorption of 100 ml/100 g or less in an aqueous medium and then oxidizing the carbon black using a hypohalite.
The above method aims at obtaining an aqueous pigment ink which exhibits excellent water-dispersibility and excellent long-term dispersion stability by oxidizing carbon black so that the surface of the carbon black has a number of active hydrogens (i.e., hydrophilic functional groups). On the other hand, since the number of hydrophilic functional groups present at the contact interface between the surface of carbon black particles and water molecules is important for dispersing carbon black in water and maintaining a stable dispersed state, it is difficult to accurately determine the dispersibility of carbon black merely based on the number of functional groups per unit weight of carbon black.
JP-A-11-148027 discloses water-dispersible carbon black which is modified by oxidation, wherein the total number of carboxyl groups and hydroxyl groups among hydrogen-containing functional groups present on the surface of the carbon black is 3 μeq/m2 or more per unit surface area. JP-A-09-286938 discloses an aqueous pigment ink in which carbon black is oxidized with a hypohalous acid and/or a hypohalite, and at least some acidic groups present on the surface of the oxidized carbon black are bonded to an amine compound to form an ammonium salt. Ammonia, a volatile amine replaced by an alkyl group having 1 to 3 carbon atoms, an amine compound replaced by an alkanol group (i.e., alkanolamine or alkylalkanolamine), or the like is used as the amine compound. Since the compounds other than ammonia have a low degree of dissociation, a substitution reaction with hydrogen in the acidic group occurs to only a small extent.